Refractive x-ray lenses have been in use for many years in a wide range of scientific and industrial applications. The focal length of a refractive x-ray lens is determined by its index of refraction, the radius of curvature of the front surface of the lens, and the radius of curvature of the rear surface of the lens.
An x-ray lens is comprised of a block of material typically having a low atomic number (Z), such as beryllium, and may be, but does not necessarily have to be, in crystal form with slots, or holes, drilled into the block of material. The focal length of the beryllium lens is directly proportional to the radius of the slot and is inversely proportional to the square of the x-ray wavelength and to the number of slots formed in the beryllium material. A single slot typically provides the beryllium lens with a focal length on the order of tens of meters. One can incrementally tune, or vary, the focal length of the beryllium lens by varying the number (N) of slots the x-ray beam passes through, i.e., the more slots through which the x-ray beam is transmitted, the shorter the focal length of the lens. At long distances it is impractical to adjust the focal length in this way because the relative focal length changes are small, while the relative difference between N and N+1 is large so the adjustment would be very coarse. In addition, while the slots in the lens material are primarily disclosed as being cylindrical in shape, they do not have to be cylindrical. Any empty space, or void, in the lens material having spaced, facing, curvilinear surfaces through which the x-rays are directed would act as a lens and would be the continuously variable by the present invention.
In many applications involving the use of x-ray lenses, it is desirable to direct an x-ray beam onto a material and vary the photon energy of the beam which is reciprocal to the wavelength over a range of energies in studying the changes in structure and characteristics of the material. Changing the photon energy of the x-rays passing through an x-ray lens results in a change in the position at which the beam is focused by the lens which is undesirable. As a practical matter, it is difficult, if not impossible, to change the number of slots through which the beam passes within the same lens to compensate for changes in the focal length of the lens arising from changes in the x-ray beam's energy.
The present invention addresses this and other problems encountered in the prior art by providing a continuously variable focal length lens for focusing an x-ray beam over a range of focal lengths. This is achieved inexpensively and in a compact manner by the present invention.